1. Technical Field
The present invention generally relates to lubricating oil compositions.
2. Description of the Related Art
Exhaust after-treatment devices, equipped on internal combustion engines to comply with emission regulations, have proven to be sensitive to the combustion by products of the fuel and lubricant used in the engine. In addition, certain types of devices are sensitive to one or more of the following: (1) phosphorus coming from the lubricant, (2) sulfur coming from both fuel and lubricant, and (3) sulfated ash resulting from the combustion of fuel and lubricant. In order to ensure the durability of the different types of after-treatment devices, special lubricants are being developed that feature relatively low levels of, for example, sulfur, phosphorus, and sulfated ash.
U.S. Patent Application Publication No. 20050043191 (“the '191 application”) discloses a lubricating oil composition having less than 2000 ppm sulfur and free of zinc and phosphorus. The '191 application further discloses that the lubricating oil composition has a minimum of 120 ppm of boron and a minimum of 80 ppm of molybdenum. Each of the examples shown in Table 1 of the '191 application disclose an ash content of 0.96, 0.99 and 1.05 for Oils 1, 2, and 3, respectively.
U.S. Pat. No. 6,777,378 (“the '378 patent”) discloses a lubricating oil composition containing (a) a base oil; (b) a molybdenum- and sulfur-containing composition derived from a basic nitrogen-containing compound, a molybdenum compound and carbon disulfide; (c) a borate ester; and (d) optionally a phosphorus-containing compound provided that the phosphorus content of the composition does not exceed about 0.10 wt. %. The '378 patent further discloses that the lubricating oil composition has a boron content of about 30 ppm to about 600 ppm and a molybdenum content of about 25 ppm to about 800 ppm.
U.S. Pat. No. 7,026,273 (“the '273 patent”) discloses a lubricating oil composition containing a major amount of oil of lubricating viscosity, and a minor amount of a boron-containing additive, a detergent additive composition and one or more co-additives. The '273 patent further discloses that the lubricating oil composition has a boron content of greater than 150 ppm, a molybdenum content of at most 1000 ppm and less than 4000 ppm by mass of sulfur.
EP 0 737 735 (“the 735 application”) discloses a lubricant composition produced by blending (a) a Mo-containing friction conditioner: and (b) a B-containing compound with a lubricant base oil. The 735 application further discloses that the lubricating oil composition has a boron content of greater than 0.015 wt % (150 ppm) and a molybdenum content of 100 ppm to 2000 ppm.
It is desirable to develop improved lubricating oil compositions which exhibit improved wear inhibition when used in an internal combustion engine.